The present invention relates to an electric ballast circuit for operating or lighting a discharge lamp such as a mercury rare gas discharge lamp, and more particularly to a discharge lamp operating circuit which make use of a condenser as a ballast and operates at a high frequency.
An example of a high frequency discharge lamp operating circuit using a condenser as a ballast is disclosed by JP-A No. 60-47398. The disclosed circuit includes a condenser and semiconductor switches and the condenser is periodically charged and discharged through a discharge lamp by ON/OFF operation of the switches so that a high frequency AC current is caused to flow in the discharge lamp. This discharge lamp operating circuit has an advantage that any inductance is not included, thereby providing possibilities of small size, light weight and low cost.
In the above-mentioned discharge lamp operating circuit, the great improvement of the efficiency of the discharge lamp may be possible by making the capacity of the condenser larger to increase a time constant CR so that the waveform of a voltage applied to the discharge lamp approaches a rectangular wave. However, such an approach results in the problem of an excessive inflow of a heating current to a filament of the discharge lamp and hence the too early blackening of the inner wall of the lamp and/or the burning-out of the filament since the decrease of impedance of the condenser and the resulting increase of the heating current are not taken into consideration.